Phase and frequency modulation diversity receiving system



c. w. HANsl-:LL 2,249,425

Filed March 27, 1940 2 Smets-sheet 1 July l5, 1941.

PHASE AND FREQUENCY MODULATION DIvERsITY RECEIVING SYSTEM c. w. HANsELl. 2,249,425 PHASE AND FREQUENCY MODULATION DIVERSITY RECEIVING SYSTEM July 15, 1941.

Filed-March 27, 1940 2 sheets-sheet 2 Patented YJuly 15, 1941 UNITED sTATEs PATENT o EElcE PHASE AND FREQUENCY MODULATION DIVERSITY RECEIVING SYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assgnor to Radio Corporation of America, a corporation of Delaware Application March 27, 1940, Serial No. 326,129

(Cl. Z50-20) 25 Claims.

munication where multipath transmission occurs,y

including radio andwirefline communication or communication by means of sound waves, me-

chanical vibrations and other means Where WaveV propagation is used. ,It is vparticularly applicable to reception of phase and frequency modulated radio waves transmitted over such distances that propagation via the earth and ionosphere above theY surface of the earth are concerned. y

Experience has demonstrated that, when signalling across an ocean or a continent Vwith radio wave frequencies from 3 to 30 megacycles' per second, it is usual for Waves to arrive at the receiver over a multiplicity of paths simultaneously. These paths occur because Waves are reflected, or refracted, up and down various numbers'of times between the ionosphere and the surface of the earth, or are refracted by various layers` of the ionosphere, in travelling from transmitter to receiver. They may also occur because Waves of different polarization with respect to the earths magnetic field travel with different velocities, or at least have different phase velocities,A inpassing through the ionosphere. There may also be Waves arriving at the receiver which have'gone round the world, not following the plane'of the e shortest great circle path from transmitter Vto receiver. Furthermore, it is observed that "the paths are continually varying in overall length no doubt in response to' variations in ionization of the upper atmosphere and in response to variations in the magnetic field of the earth above its surface, and 'perhaps also in responsel to unrecognized factors. The multiple wave paths and the continual variation in the paths give rise to fading, selective frequency fading and distortions of the modulations in the'resultant'waves which supply the input current to a receiver.

-The invention is applicable to radio communications systems operated over relatively short distancesA Where the modulation frequencies range up to values sohigh that differences in multiple path lengths become equal to `or greater than an appreciable fraction of the wave length ofia modulation wave.

Such conditions are'encountered in television transmission'A where l modulation frequencies used at present may range up to 5 megacycles, corresponding to a modulation wave length of 60 meters. In this case multi-path distortion will begin to be apparent when multiple paths differ inlength by as little as say l0 meters, or about 33 feet. EX- -perience has shown that path length differences of 33 feet and more are very Ycommon in radio communication over short distances, particularly in metropolitan areas where reiiections from buildings cause numerous multiple paths.

In the case of television between fixed transmitters and receivers the relative lengths of multiple paths do not usually change` rapidly but they do change due to effects of weather, rain orsnow falling on reflecting surfaces, etc. When either the transmitter 'vor receiver are mobile units, however, the multiple paths may vary greatly Aand rapidly with time. kThe present in- Vention is particularly applicable to reception of signals transmitted by mobile transmitters, for

example transmitters in motor trucks and airplanes,

The invention is also applicable to submarine signalling by means of audible or superaudible vibrational waves, which may be reected from various objects in a manner to cause multiple paths between transmitter and receiver. It is applicable to some kinds of wire line communication, particularly in carrier ,current systems where multiple paths due to line reiiectionsand other causes may exist, and in other places. v

Thus,rit will be apparent that the invention is applicable to any system of signalling where transmitted currents arrive at the receiver inthe form of a plurality of components which have required different time intervalsfor propagation from transmitter to receiver, particularly if the time delays over the several paths are variable.

A primary object of the invention is to reduce the effects of noise and interfering radiations upon the signals or modulations which it is Vdesired to receive. Y

Another primary object of the invention is to reduce ,the distortions yin the received signal modulations brought about by the interactions of the multiple received Waves.

'Another object is to provide improved means for accomplishing the foregoing objects.

Prior knowledge and experience with radio ciri cuits had demonstrated that, when multiple paths lare present, the manner of combination of multiple path currents may be greatly altered by moving the receiving antenna only a relatively` short distance. 'Furthermore if two spaced anconsequence is that the overall resultant carrier Wave signal strengths, observed in the input of a number of receivers operated'with spaced antennas, will vary in a more or less random manner.

A similar situation exists in antennas located close together if these antennas are designed to collect energy of different polarizations.`

The effect of interfering noise from external sources is less in a receiver which has a relatively large resultant carrier wave signal strength at the receiver input terminals. Assume, forexample, that noise is due to the carrier wave of an interfering transmitter which beats with the carrier wave of the transmitter we wish to receive. The interfering carrier current produces an equivalent phase modulation of the useful carrier current with a peak angular phase deviation equal to the angle whose sine is the ratio of the strength of interfering current to the useful (Wanted) signal carrier current. If the useful carrier current strength varies over a range of 10 to 1 or more then the interference may also vary over a range of about l0 to 1 in current, or 100 to 1 in power.

If the (relative) interfering current strengthy increases until it exceeds the desired current in amplitude then the peak phase modulation beat in the receiver input and output passes a .limiting value of plus and minus 90 degrees but use-- ful phase or frequency modulation of the desired carrier wave is suppressed and` appears only as a frequency variation of the beating frequency between the two carrier currents.

Other external noises, and receiver noises such as those produced by thermal agitation in the receiver input circuits follow substantially the samelaws as the beat from an interfering transmitter, if we take into account the instantaneous resultant amplitude of the noise with respect to the carrier amplitude. That is, the resultant peak noise in the output of the receiver will be equal to that which would be produced by a peak carrier current phase modulation having an angular deviation equal to the angle whose sine is equal to the ratio of instantaneous resultant radio frequency noise currents divided by the carrier current. The noise, if and when'its-instantaneous amplitude exceeds the amplitude of the useful carrier, suppresses the useful modulation of the carrier Wave in substantially the same manner as an overwhelming interfering transmitter carrier.

Because of the existence of the phenomena described it becomes important, lin a phase or frequency modulation diversity receiving system to obtain the greater part of the final useful signalling current from the receiver, or receivers, having the strongest carrier current input. By this means an optimum signal to noise ratio is obtained.

This has not been obvious in the past and was not disclosed in my prior United States Patent No. 1,803,504 because the useful signal output fromra phase or frequency modulation receiver, 7F;

so long as there is a useful output, is independent of the strength of the received carrier Wave. The useful receiver output current is determined by the phase or frequency, and the variations in phase or frequency, of the received carrier current and not by the amplitude or variations in amplitude, of the current.

It was known that the useful output current of an amplitude modulated receiver varied in proportion to the input current and that the signal to noise ratio varied correspondingly.

Beverage, Peterson and Moore used this observation in devising their amplitude modulation diversity receiving systems described in United StatesPatents Nos. 1,863,695, 1,874,866, 2,004,126,

y2,604,127 and 2,004,128.

My present invention uses the observation that, although the strength of useful signal output from a phase or frequency modulation receiver is independent of the strength of input carrier current, for any adequate signal input level, the noise varies inversely with variation in input carrier current.` It is important, for obtaining optimum signal to noise ratio, in a phase or frequency modulation diversity receiving system to derive the greater part, if not all, of the useful output from the receiver, or receivers, which have maximum carrier current input and to provide automatic means for reducing the output from a receiver, or receivers, in which the input carrier current is low. This is vbecause when the input carrier is low, the signal to noise ratio is unfavorable. The desired overall result isan output current which is independent of input current strength variations but in which the output current contributions of the several receivers are controlled to make the receiver, `or receivers, with strongest input current predominate. My present invention accomplishes this result in a desirable manner. Y y

In addition to reducing the effectsof noise, the invention is also effective in reducing distortions of the received signals. This reduction in distortion involves phenomena which apparently hasnot beengenerally understood in the art.

One means by which distortion is reduced is that, so long as the output from a phase or frequency modulation diversity receiving system comes from receivers in each of which the several multi-path alternating-frequency currents are adding with phase relationsv such as to produce a large resultant current, there is a relatively small probability that the useful modulation will result in amplitude variations which carry the resultant current below the noise. Therefore, there is less probability of gaps and breaks in the useful modulation.

Another reduction in distortion results from the decreased phase modulation beats produced by any one component current when that current responds to the useful modulation with a difference in time, due to difference in path length, with respect to the resultant current. In general, paths of most nearly equal length will supply the receiver with currents of most nearly equal strength. Whenseveral strong component currents add together to produce a large resultant, there is a good probability that the time difference vin arrival of their modulations is relatively small and their'modulations will be additive up to relatively high Vmodulation frequencies. Simultaneously there may be other current components, modulations of which arrive earlier or later than the main components.V These earlier and later components do not change phase or i livered to a variable gain amplifier 26.

frequency at the same time as the said resultant current and so, during modulation, given lrise to phase modulating beating which represents a distortion or introduction of spurious modulation. So long as the early or -late currents beat with a strong resultant current they produce relatively small spurious phase and frequency modulations. However, if the resultant current is weak, they may produce large spurious modulations. Consequently, for reducingV spurious modulations, it is important to make predominant, in supplying the useful output, those receivers in a diversity system .which have high input currents.

In Fig. 1 I have shown one form of phase or frequency modulation diversity receiving system in accordance with my invention. This figure shows three phase or frequency modulation receivers, with separate spacedv antennas, for supplying a common modulation frequency output circuit. All of the elements shown in the system are devices for performing indicated functions which are well known in the art and for which one skilled in the art may readily supply numerous alternative detail devices. In Fig. 1 separate oscillators are used in each receiver to heterodyne the signal modulated wave to an intermediate or lower frequency for amplification and detection. In Fig. 2, which is otherwise sim'- ilar to Fig. 1, a single oscillator connected with the rst detectors in each receiver is used.

Tracing the signals through any one of the receivers, We note that an antenna A picks up power at the frequency of the signalling waves and delivers this power through a transmission line L to a frequency selective high frequency amplifier l in which the power levelV is increased. Output from the high frequency ampliV` fier I0 enters a heterodyne detector I4 where'it beats with input to the detector from a heterodyne oscillator i6. Output from the h'eterodyne detector I 4 appears at a lower, or intermediate, carrier frequency in accordance with the well known action of superheterodyne receivers'.

' Intermediate frequency power is further ampliiied in a frequency selective amplifier I8 and delivered to a frequency selective amplitude limiter where all variations in strength of the received carrier wave are eliminated, so long as the received power is above some arbitrary low minimum value. The output from the limiter is then intermediate frequency current power V-having substantially no amplitude modulation but having retained in it all the phase or frequency modulation.

The amplitude limited current is delivered to a phase or frequency modulation detector 24 out of which modulation frequency currents are de- Output from the variable gain amplifier 26 is delivered to a common output circuit 36 for all three receivers after which the three combined output energies may be further amplified by a combined amplifier 34 and utilized for any intended useful signalling purpose, such as, for example, supplying a program to a radio broadcasting network over a system of wire lines or radio relays.

In order to control the gain of an amplifier in each receiver in accordance with the carrier amplitude at the input of the remaining receivers,

I provide two rectiiiers 60 and 62 Ahaving inputs connected to the output of intermediate frequency amplier A I8 so that they are excited by wave energy that is of an amplituderwhich is representative of the amplitude of the wave energy at the antenna A. Each rectifier supplies adirect-current Voutput potential of a polarity as shown. The other receivers include similar rectiers as designated at 6D', B2', and 60, 62". The direct-current output of rectifier 60 is in series with the direct-current output of 62 and these two'outputs in series are supplied to the gain control elements 10 of the receiver fed by antennaV A". This circuit comprises the outputs of jrectiiiers 60, 62', lead 8U" from the negative side off-the series connection, lead 84, common lead 86 V'the direct-current output of a rectifier |00, the purpose of which will be described hereinafter and through the common lead 90 at the positive side of the direct-current output of rectifier |00 to the other side of gain control elements 10.

The direct-current outputs Vof rectiers 60" and 62 in series are connected to gain control elements 16' through leads 80', 84, common lead 86,' rectifier |00, output and common lead 90.

The direct-current outputs of rectifiers 6U' and 62 in series are connected to gain control elements 'l0 through leads 80, 84', common lead 86, the output of rectifier i60 and common connection 90.

Thus, each pair of receivers supplies control potentials to the remaining receiver. For eX- ample, the gain of the amplifier of 2E is a functionof the output of the intermediate-frequency amplifiers I8' Vand I8. The gain of the amplifier of stage 26' is a function of the amplitude at the outputs of intermediate-frequency amplifiers lland I8", etc.` The gain of each amplifier 26, 25. and 26" is independent of its own output as far Vas the gain control circuits'described up to this point are concerned.

In operation of the system, in accordance with my invention, each of the three receivers is supplied with an vautomatic gain control for its modulation frequency variable gain amplifier. However, unlike ordinary automatic volumejcontrol systems in single receivers, in thepresent case automatic volume control current for `each receiver is obtained by 'rectifying a portion of the intermediate frequency Acurrent power of the other vtwo receivers, before amplitude limiting of the intermediate frequency current. 'I'he automatic volume control applied in eachreceiver is such that strong intermediate frequency currents in the other receivers will reduce the amplification of each receiver. Then any one receiver, or pair of receivers, suppresses or reduces output from the other receiver and the combined modulation frequency output current is obtained predominantly from the receiver, or receivers, having greatest carrier current input.

' Preferably, the automatic volume control c urrents to the several receivers is supplied through time'constant circuits T0, 10 and lil having time yof response to current variations which is about equal to, or greater than, the time of a half cycle of the lowest useful modulation frequency so that the automatic volume control circuits Will not respond to amplitude modulations of the received signals within the useful modulation Vfrequency range.

u The arrangement of Fig. l, as so far described, accomplishes the one very desirable result of making the output from a phase or frequency modulation diversity receiving system `come predominantly from the receiver, or receivers, in the combination which have strongest signal carrier input currents. Therefore, a greatly improved signal-to-noise ratio and a reduction in modulation distortion will be obtained.

However, the system as so far described suers from another operating defect which requires correction. If the input currents to all the receivers should either increase or decrease at once the combined output signal current would be changed. Therefore, we have lost at least some of the advantage in a phase or frequency modulation system which lies in the fact that .the strength of receiver output current is substantially independent of the strength ofthe input current. Some means must be provided to overcome this defect, without destroying the advantages of the system as so far described. My arrangement as illustrated in Fig. l, provides this additional feature.

In the arrangement of Fig. l, I have provided for a continuous pilot signal phase or frequency modulation at a frequency above or below the band of frequencies utilized for the useful modulation. Preferably, the pilot signal is at a frequency above the band of the useful modulation. If the useful modulation lies within the audible range of say 15 to 15,000 cycles per second then I prefer to operate the pilot signal at an inaudible frequency of say 20,000 cycles per second. The pilot signal is applied at the receivers.

In Fig. l, I have indicated an oscillator 40 which supplies energy to phase or frequency modulate the heterodyne oscillators I6, I6 and I6 of the three receivers at say 20,000 cycles per second. This results in a 20,000 cycle phase or frequency modulation of the intermediate-frequency current produced in the output of the heterodyne detector I4 and as a consequence 20,000 cycle currents appear continuously in the output of the whole diversity receiver system while the useful signal is present. The 20,000 cycle current is taken out through a frequency selective filter 50 and rectified in a rectifier |00 to provide a direct current, or potential, approximately proportional to the strength of the 20,000 cycle current. The direct current is utilized as an additional automatic gain control for the variable gain modulation frequency amplifiers and is so connected and adjusted that it tends to hold itself, and the 20,000 cycle current nearly ccnstant. In holding itself nearly constant, through the automatic gain control system, the 20,000 cycle pilot current also holds the overall useful phase or frequency modulation response of the receiver nearly constant.

The overall result is a diversity receiving system for phase or frequency modulated waves in which the strength of modulation frequency output is substantially independent of the combined strength of carried wave input but in which individual receivers with strongest relative input contribute larger relative portions of the output. As a result, a better average signal-to-noise rati and less distortion of the desired signals is secured than was possible with the system described in United States Patent #l,803,504.

'I'he system shown in Fig. 1 is capable of numerous modifications for obtaining substantially similar results. In particular, the variable gain amplifiers 26, 2E and 25 may be operated at the intermediate frequency instead of at modulation output frequency provided they are placed after the limiters 20, 20 and 20". In Fig. 2, I have illustrated this modification. Here the limiter 20 supplies phase or frequency modulated wave energy to an intermediate-frequency stage 2| wherein the gain control is included. It is essential only that all signal frequency amplitude variation or modulation of the received signal be suppressed before the signal reaches the automatic gain controlled amplifier. Of course, I use intermediate-frequency amplification between the limiter 20 and demodulator 26 of the modification of Fig. 1 where necessary.

Although I have shown three separate heterodyne oscillators for the three receivers, which has practical operating advantages, I might employ a single oscillator for supplying the input to all three heterodyne detectors. This modification is illustrated in Fig. 2 wherein the singlesource I6 modulated as t-o phase or frequency by oscillations from oscillator 40 supplies beating oscillations to all of the rst detectors in units I4, Il and I4.

What is claimed is:

1. In a diversity signalling system for reducing the effects of fading on wave length modulated wave energy being received, a plurality of modulated Wave receivers each including signal Wave energy pick-up means, Wave amplitude limitingv means coupled'to said pick-up means, and amplifying means of controllable gain connected-to said limiting means, an automatic gain control means connected with each of said amplifying means for controlling the output of each of said receivers in accordance With the amplitude ofthe wave energy picked up by the other of said receivers, and a common output circuit connected with all of said receivers.

2. In a diversity signalling system for reducing the effects of fading on wave length modulated Wave energy being received, a plurality of modulated Wave receivers each including wave amplifying means of controllable gain on which said modulated wave energy may be impressed, means connected with said amplifying means for controlling the gain of a plurality thereof in accordance withfthe wave energy amplitude impressed on one of said amplifying means, a common output circuit, and Wave length modulated wave demodulating means coupling each of said amplifying means to said common output circuit.

3. In a diversity signalling system in combination, a plurality of receivers each including signal pick-up means, Isignal amplitude limiting means, and signal amplifying means of controllable gain connected in cascade in the order recited, automatic gain control means excited by the signal and connected with said amplifying means of controllable gain for controlling the output of each of said receivers in accordance with the amplitude of the signal picked up by the other of said receivers, and a common output circuit connected with all of said receivers.

4. In a diversity signalling system, a plurality of modulated wave receivers each including wave amplifying means, wave amplitude limiting means and wave amplifying means of controllable gain connected in cascade in the order recited, means for impressing wave length modulated wave energy on said first amplifying mean-s, means in each receiver connected with said first amplifying means and said amplifying means of controllable gain in a plurality of said receivers for controlling the gain of a plurality of said receivers in accordance with the wave amplitude in said rst amplifying means in one of Isaid receivers and a common output circuit connected with all of said receivers.

5. In a wave length modulated wave system a plurality of receivers each including modulatedl wave amplifying means and wave amplitude variation demodulating means, said amplifying means including a variable gain amplifier, means-for impressing modulated Wave energy on each of said demodulating means, means interconnecting the demodulating means and variable gain amplifiers of said receivers for controlling the gain of a plurality of the receivers in accordance with thev output of the demodulating means of that receiver the input of which is excited by the strongest modulated Wave, a Wave length modulated Wave demodulated in each receiver, and a common output circuit connected with all of said last nameddemodulators.

6. In a diversity receiver system a plurality of receivers Whose outputs are substantially independentv of input amplitude variations for any adequate signal input level, amplifying means of variable gain and rectifying means included in each of said receiving means-and gain control circuits interconnecting the rectifying means and the variable gain amplifying means in said receivers in such a manner that that receiver having the highest input level controls the gain of all of the remaining receivers...v

7. In a diversity receiver system, a plurality of receivers, gain control means for biasing each of a plurality of said receivers to reduce their output to a negligible value in the presence of strong signals in another one of said receivers, means for combining the outputs of all of said receivers, and additional means for con- .trolling the gain of all of said receivers in accordance with said combined output to maintain said combined output substantially constan-t irrespective of signal input level.

8. In a diversity system for the reception of high frequency waves the frequency or phase of which have been modulated in accordance with signals, a plurality of receivers including amplifiers of controllable gain Whose outputs are substantially independent of input amplitude variations for any adequate signal input level, rectifying means included in each of said receiving means and gain control circuits interconnecting the variable gain amplifiers in said receivers in such a manner that that receiver having the highest input level controls the gain of all of the remaining receivers.

9; In a diversity receiver system for the reception of high frequency Waves the frequency or phase of which have been modulated in accordance with signals, a plurality of receivers each including Wave amplifying means,.vvave amplitude limiting means and phase or frequency modulated Wave demodulating means, said amplifying means including an amplifier of variable gain following said Wave amplitude limiting means, a rectifier in each receiver excited by received Waves and means interconnecting the rectifiers and variable gain amplifiers of said receivers for controlling the gain of all of the receivers in accordance with the output of the rectiiier in that receiver the input of which is excited by the strongest carrier Wave and a common output circuit connected with all of said demodulating means.

10. -In a diversity receiver system for the re- Aception of phase or frequency modulated wave energy with simultaneous reduction of the effects offading thereon, a plurality of receivers each including Wave amplifying means of the hetero dyne type including a first detector and a source of oscillations coupled thereto, amplitude limiting-'means coupled with each of said detectors, phase or frequency modulated wave demodulating, means coupled With each of said limiting means, an amplifier of variable gainin one Y of said couplings, a common output circuit coupled Withlall of said demodulating means, a pluralityof rectifyingV means connected with the output of each of said heterodyne detectors, the number of said rectifying means for each of said receivers being'one less than the number of receiversin said entiresystem, means connecting the output of the rectifiers of each of said receivers to the variable gain amplifiers in the remaining receivers, means for impressing additional oscillations on the;.intermediate-frequency Wave energy in the output of said first detector in each receiver, means -at the output of said phase or frequency modulated wave demodulator means for selecting therefrom said additional oscillations and means for controlling the gain of all of said receivers in accordance with the mean amplitude of said additional oscillations so selected.Y

11. In a diversity radio receiving system for phase and frequency modulated waves, a plurality of separate receivers each including a Wave amplifier oficontrollable gain, a common output circuit coupled to all of said receivers, means for impressing phase or frequency modulated waves on each of said receivers, and means for reducing the effects of noise and modulation `distortlons-in .said receivers comprising automatic means interconnecting said amplifiers and excited by the Waves being amplified for obtaining greatest signal output from that receiver or receivers With greatest signal input.

12, In a diversityradio receiving system for phase and frequency modulated waves, a plurality of separate receivers each including an amplifier of variable gain, means for impressing phase'or frequencymoduiated Wave energy on each receiver, an output circuit'wherein the .outputs of all of :said receivers are combined, coupled to all of said receivers, means interconnecting said ampliers ,of variable gain `and excited by the modulated wave energy for obtaining the greatest portion of the combined signal from the receiver or receivers having the greatest Wave energy input, and automatic means energized by the combined outputs of said receivers for additionally controlling ,ther gain of said amplifiers, whereby their combinedsignal output is substantiallyy independent of the overall power gain in the receivers.A

13. In a radio receiver comprising signal modulated Wave amplifying and demodulating means, means for maintaining substantially constant the ratioy of percentage modulation of Wave power input to signal power output in said means comprising means for modulating the Wave energy in the amplifying means ofthe receiver -by a modulation frequency outside tne band of signal modulation frequencies, and means for separat-l ing this modulation energy from the signal frequency energy inthe voutput of the receiver and utilizinglit to .maintain itself, and the overall gainof the receiver, substantially constant.

le. .Means for overcoming variations in the ratio of percentage carrier Wave modulation in the input of alcascaded amplifier including a stage of variable gain to modulation output power in the output of said amplifier comprising, means for impressing voltages on the input of said amplifier of a frequency outside of the frequency of the signal modulations on said Wave, means Y for deriving potentials characteristic of said impressed voltages from the output of said amplier, and means coupled with,

said last named means and with said stage of variable gain for controlling the gain of said system in accordance with said potenti-als.

15. In a wave length modulated wave signalling system, a plurality of receivers each includingV wave amplifying means, Wave amplitude Variation demodulating means and Wave length modulated wave demodulating means, said amplifying means including -a variable gain amplifier, means for impressing modulated wave energy on each of said demodulating means, means interconnecting the said wave amuplitude variation demodulating means of each receiver with the said amplier of variable gain of other of the receivers for controlling the gain of a plurality of the receivers in accordance with variations in the amplitude of the received wave in that receiver the input of which is excited by the strongest wave, and a common output circuit connected to all of said Wave length modulated Wave demodulators.

16. In a diversity receiving system for the reception of wave length modulated wave energy, a plurality of wave length modulated Wave receivers each including Wave pick-up means, wave y between the pick-up means and the wave length modulated demodulating means, a common output circuit, a coupling between each of the wave length modulated Wave demodulating means `and said common output circuit, Wave amplifying means of variable gain in one of said last twoY couplings, and means coupling the wave amplitude variation rectifying means in each receiver to the amplifier of variable gain in the remaining receivers to control the gain of a plurality of the receivers in accordance with the output of the rectifying means in one receiver.

17. In a diversity receiving system for the reception of wave length modulated Wave energy, a. plurality of wave length modulated wave receivers eachv including wave .pick-up means coupled to Wave amplitude limiting means, Wave length modulated wave energy demodulating means, and wave amplitude variation rectifying.

means, a coupling between the pick-up means'and the rectlfying means, a coupling between the :1 amplitude limting means and the wave length4 modulated demodulating means, a common output circuit, an ampliiier of variable gaincoupling each of the Wave length modulated wave demodulating means to said common output circuit, and means coup-ling the Wave amplitude variation,V

rectifying means in each receiver to the amplifier of variable gain in the remaining receivers. to control the gain or a plurality of theireceivers in accordance With the output of the rectifier in one receiver.

18. In a diversity receiving .system for the re. ception of modulatedwave energy, a plurality of modulated Wave receivers each including, modulated wave pick-up means, modulated waveenergy demodulatingv meansVmeans Acoupling saidl receivers, and rectifier means coupled to the ampliiiers of variable gain and excited by output energy in said common circuit for controlling the gain of all of said amplifiers of variable gain.

19. In a diversity receiving system for the reception of wave length modulated wave energy, a plurality of receivers each including wave renergy amplifying means, an amplitude limiter coupled to said amplifying means, a wave length modulation detector coupled to said 'limiting meansa variable gain amplifier coupled to said wave length modulation wave detectora common output coupled to the variable gain amplifier in Veach receiver, wave rectifying means coupled to the amplifying means in each receiver, means coupling the rectifying means of each receiver to the amplifier of variable gain in the remaining receivers to control the output of the remaining receivers in accordance with the amplitude ofthe wave energy in the amplifying means of each receiver, and means for controlling the gain of all of the receivers in accordance with the amplitude of the combined receiver outputs in said output circuit.

20. In a diversity receiving system for the reception of wave length modulated wave energy, a plurality of receivers each including wave energy amplifying means, means for impressing modulated vvave energy-on said amplifying means, an

-amplitude limiter coupled to said amplifying means., a variable gain amplifier coupled to said amplitude limiter, a wave length modulation detector coupled to said variable gain amplifier, a common output circuit coupled to the last vnamed amplifier in each receiver, wave rectifying means coupled te the amplifying means in each receiver, means coupling the rectifying means of each receiver to the amplifier of variable gain in the remaining receivers to control the output of the remaining receivers in accordance with the amplitude of the wave energy in the amplifying means of each receiver, and meansv for controlling the gainl of all of the receivers in accordance with the amplitude of the combined receiver outputs in said output circuit.

21. Ina 'diversity receiver system a plurality of receivers whose outputs are substantially independent of input amplitude variations for yany adequate signal input level, amplifying means-of variable gain and rectifying means included in each of said receiving means, said amplifying meansl and i'ectifying means each having'an in` put excited by signals to be received, andfgainv control circuits interconnecting the rectifying means and the variable gain amplifying means iny said receivers in such manner that that receiver having the greatest input signal power provides a correspondingly greatest output power.

l22. In a diversity receiver system a plurality of recei`vers,means for impressing Ysignals on eachv receiver,'gain controlled meansexcited by said signals for reducing the voutput of each receiverl in the presence of' stronger input signals to other receivers., means for -vcombining the outputs of all of said receivers, and additional means for controlling the gain of all of the receivers in accordance with a'portion of the combined output to .make the combined' output substantially independent of input signal power.

23. In a diversity system for .the reception of signals sent by phasevor `frequency modulation of waves, aplurality vof receiversincluding amplifiers of. controllable gain'whose outputs are substantially 'independent 'of inputrpower for 'any adequate received power level, a commonoutput for all of said receivers, means for impressing signals on each of said receivers, means excited by said signals to derive currents which vary as the input signal power varies and means interconnecting said last named means and said amplifiers of controllable gain to utilize these currents to control the gain of the ampliers of controllable gain in such manner that the combined output from the system is substantially independent of input signal power and is derived principally from the receiver, or receivers, of greatest relative input signal power.

24. In a diversity receiver system for the reception of signals transmitted by frequency or phase modulating a carrier wave, a plurality of receivers each including a heterodyne detector, means for impressing signals on said heterodyne detector, an amplifier which removes signal amplitude modulations coupled to said heterodyne detector, a demodulator coupled to said ampli- Iier, a common output circuit,a coupling between said demodulator and said common output circuit, a variable gain amplier in one of said last named couplings, means excited by said signals to derive a current which varies with variation in input power and means interconnecting said last named means and said amplifiers of variable gain to control the gain of each Variable gain amplifier by currents derived from other receivers in such manner that the receiver with greatest input signal power delivers greatest output power tosaid common output circuit.

25. In a diversity receiving system for reception of signals sent by phase or frequency modulating a carrier current, a plurality of receivers each including aheterodyne detector, means for impressing said signals on said detector, means to suppress amplitude modulations without suppressing phase or frequency modulation, a coupling between said last named means and said detector, a phase or frequency modulation demodulator coupled to said last named means, an output circuit coupled to said demodulator, a variable gain amplier in one of said last two couplings, and means connected with said amplier for automatically controlling the gain of the variable gain amplifiers of all receivers in such manner that the combined output of the receivers is substantially independent of the total carrier current input power to the receivers and the portion of output power furnished by each receiver is greatest when that receiver has greatest relative input power.

CLARENCE W. HAN SELL. 

